Experimental study on engineering properties of fiber-stabilized carbide-slag-solidified soil

Carbide slag has been used to prepare solidified soil to effectively reduce the stacking and disposal of carbide slag and achieve efficient resource utilization. Because of the significant brittleness and low strength of carbide-slag-stabilized soil, fibers were added to carbide-slag-stabilized soil in this experimental study. The effects of fiber length and fiber content on the unconfined compressive and indirect tensile strengths of carbide-slag-stabilized soil were investigated. The concepts of the density of fibers in solidified soil and the number of fibers in a unit volume solidified soil were proposed, and the effects of fiber distribution density on the mechanical properties of the solidified soil were evaluated. The fibers increased the indirect tensile strength of the carbide-slag-solidified soil, which was significantly higher than the unconfined compressive strength of the solidified soil. The fibers had no significant effect on the unconfined compressive and indirect tensile strengths of the 7 d carbide-slag-solidified soil but increased those of the 28 d carbide-slag-solidified soil. The enhancement effect was the most significant when a 0.3% content of 19 mm long fibers was incorporated into the carbide-slag-solidified soil.

Yes -all data are fully available without restriction

29
The efficient treatment and utilization of carbide slag is the only approach to solve the 30 environmental pollution problem of the carbide industry and achieve sustainable 31 development. Many researchers have attempted to utilize calcium carbide slag for soil 32 reinforcement [1][2][3], and the solidified soil has problems, such as low strength and toughness and 33 susceptibility to cracking [4]. After more than four wetting-drying cycles, the compressive strength 34 of the carbide-slag-stabilized soil fails to satisfy the strength requirements of the subgrade [5,6]. 35 Using fibers to stabilize soil is an effective soil reinforcement technique. Fibers can 36 significantly increase the strength and toughness of soil and solve soil cracking problems [7,8]. The 37 addition of fibers effectively improves the tensile strain and reduces the cracking of specimens 38 shear strength, and deformation resistance of cement-clay [11][12][13]. The incorporation of fibers had 41 obvious effect on delaying the cracking of carbide slag stabilized soil specimens under load. 42 [14]. The reinforcement effect of the fiber depends on the strength of the interface between the 43 reinforcement and soil, and the mechanical interactions between the reinforcement and soil are 44 Hebei Province, China. The study did not involve private or protected land. According to Test 68 Methods of soils for Highway Engineering (JTG 3430-2020) [20], the physical indexes and particle 69 gradation of test soil are determined. The physical indexes are listed in Table 1, and the particle size 70 distribution is shown in Fig 1,  Carbide slag was purchased from Dezhou Shihua Chemical Co., Ltd. The raw calcium 75 carbide slag had a high moisture content and a peculiar smell. After drying and dehydration, it was 76 crushed by soil milling and passed through a 0.075 mm square hole sieve. The carbide slag under 77 the sieve was used for the tests (Fig 2). The main components of the calcium carbide slag are listed 78 in Table 2. The carbide slag contained a large amount of calcium oxide similar to that of quick lime 79 and exhibited significant activity (Table 2). 80 Table 2. Chemical Composition of Carbide Slag. 81 The fibers were purchased from Changsha Ninxiang Building Materials Co., Ltd. 82 Polypropylene fibers with lengths of 6, 12, and 19 mm were used for testing (Fig 3). According to 83 and mechanical properties of the fibers are listed in Table 3. 86 Table 3. Physical and Mechanical Properties of Polypropylene Fibers. 87

88
This study was divided into three steps to investigate the effects of the fiber length and 89 content on the performance of fiber-stabilized carbide-slag-solidified silt, optimal fiber length and 90 content, and effect of fiber distribution density on the mechanical properties of the solidified soil.  The specimens were prepared using the method specified in "Inorganic Binder-Stabilized 102 Material Specimen Production Method (Cylindrical)" (T0843-2009). Dried sieved carbide slag and 103 silt were thoroughly blended at a mass ratio of 2:8. An appropriate amount of the fiber was 104 proportionally added to the mixture and thoroughly stirred manually to fully disperse the fiber into 105 the mixture. An appropriate amount of water was added to the mixture blended with the fiber, stirred 106 evenly, and placed in a plastic bag. The bag was sealed and placed in the dark for 24 h. A 50 mm × 107 50 mm test mold was adopted, and the calculated weight of the soil sample was determined based 108 on the degree of compaction (96%). Twelve samples were prepared for each group. After the 109 specimens were produced using the static pressure method, they were placed in a curing box 110 (temperature = 20±2 °C; relative humidity ≥ 95%). The specimens are shown in Fig 4. The 111 proportions of the specimens of fiber-stabilized carbide-slag-solidified soil are listed in Table 4. 112 According to the experiment, the optimum moisture content of 2:8 carbide-slag-solidified 125 soil was 13%-14%. First, five portions of 2:8 carbide-slag-solidified soil were prepared, and the 126 target moisture contents were 10%, 12%, 14%, 16%, and 18%. Compaction tests were performed 127 according to the compaction test method for inorganic binder stabilization materials (T0804-1994). 128 The compaction curve of the soil samples is shown in (Fig 5). The optimum moisture content of 2:8 129 carbide-slag-solidified soil was 13.62 %, and the maximum dry density was 1.68g/cm 3 . Because 130 the amount of fiber was small, its effects on the optimum moisture content and maximum dry density 131 were not evident. Therefore, when preparing an unconfined compressive strength test and indirect 132 tensile strength test samples, it could be directly configured according to the optimum water content 133 of 13.62 % and the maximum dry density of 1.68g/cm 3 .

174
Because fibers are stable materials, they do not react with silt or calcium carbide slag, and 175 their effect on soil is similar to the mechanism by which plant roots solidify in soil. According to 176 the bending mechanism, the bending fibers between soil particles limit the change in the relative 177 position of the soil particles through the pressure and frictional force on the concave side of the soil 178 particles, stabilizing the soil. According to the intercrossing mechanism, the intercrossing fibers in 179 soil form a spatial network structure, and the action of external forces in one direction is 180 counteracted by the entire network structure. In addition, the incorporation of fibers effectively 181 reduces the internal pores of solidified soil, densifies the soil, and improves its mechanical 182 properties; this can be called the "filling mechanism." Based on the bending, intercrossing, and 183 filling mechanisms, the preconditions for fiber stability in the soil are that the fiber must have a 184 sufficient length and minimum distribution density in the soil. cementation reaction between the carbide slag and soil particles occurred, and the newly generated 207 reaction products adhered to the spatial network structure formed by the fibers, limiting soil 208 deformation and increasing the unconfined compressive strength of the soil. For the same carbide 209 slag content and soil quality, the intercrossing mechanism effect became more significant with 210 increasing fiber content and length. Moreover, the spatial network structure stabilized further, and 211 the increase in the unconfined compressive strength of the solidified soil became significant. 212 However, when the fiber content exceeded a specific value, the fibers were less uniformly 213 distributed in the soil, and the spatial network structure became less stable. indirect tensile strength of the 7 d solidified soil was observed for the fiber length of 19 mm (Fig 9). 223 When the added amount was 0.3%, the indirect tensile strength of the solidified soil increased by 224 15.38% compared with that of the solidified soil without fiber. However, when the fiber content 225 was 0.4%, the indirect tensile strength of the solidified soil decreased. This trend occurred because 226 when the fiber length was extended, a high quantity of the fiber in the stabilized soil was difficult 227 to distribute uniformly, easily formed clusters, reduced the bending effect, and intertwined. For the 228 28 d solidified soil, cementing between the carbide slag and soil particles was relatively complete, 229 and adequate bonding between the fiber and solidified soil was achieved. Therefore, the fiber 230 significantly increased the indirect tensile strength of the 28 d solidified soil. The maximum indirect 231 tensile strength of the 28 d solidified soil was observed using 19 mm long fibers. When the added 232 amount was 0.3%, the indirect tensile strength of the solidified soil was 76.64% higher than that of 233 the solidified soil without fibers. However, when the fiber content was increased to 0.4%, the 234 indirect tensile strength of the solidified soil decreased, mainly because of difficulty in achieving 235 uniform distribution with a relatively high amount of extended fibers; this easily induced the cluster 236 phenomenon, reduced the bending effect, and caused intertwining.  Under the same conditions, the unconfined compressive strength of the 7 d solidified soil 256 slightly increased with the fiber content (Fig 10). The unconfined compressive strength of the 257 solidified soil reinforced with 19 mm fibers was the highest but only 7.14% higher than that without 258 fibers. The main reason for this was that for the short curing period, the fibers did not form an (1) The fibers did not significantly increase the unconfined compressive and indirect 288 tensile strengths of the carbide-slag-solidified soil at 7 d but effectively increased those at 28 d. 289 When 19 mm long fibers were incorporated into the carbide-slag-solidified soil, the effect became 290 most significant for the fiber content of 0.3%. 291 (2) The increase in the indirect tensile strength of the carbide-slag-solidified soil by the 292 fibers was significantly higher than that in the unconfined compressive strength. 293

378
The authors declare that no competing interests exist.

Supporting Information
Click here to access/download Supporting Information supporting information.docx

29
The efficient treatment and utilization of carbide slag is the only approach to solve the 30 environmental pollution problem of the carbide industry and achieve sustainable 31 development. Many researchers have attempted to utilize calcium carbide slag for soil 32 reinforcement [1][2][3], and the solidified soil has problems, such as low strength and toughness and 33 susceptibility to cracking [4]. After more than four wetting-drying cycles, the compressive strength 34 of the carbide-slag-stabilized soil fails to satisfy the strength requirements of the subgrade [5,6]. 35 Using fibers to stabilize soil is an effective soil reinforcement technique. Fibers can 36 significantly increase the strength and toughness of soil and solve soil cracking problems [7,8]. The 37 addition of fibers effectively improves the tensile strain and reduces the cracking of specimens 38  and compressive resistance of reinforced soil and reduce the sensitivity of soil to water [19]. 49 Currently, research on the effects of fiber length and fiber content on the properties of fiber-50 stabilized carbide-slag-solidified soil has not been conducted extensively. 51 In this study, fiber-stabilized carbide-slag-solidified silt was adopted as the research In this study, the properties of fiber-carbide slag-solidified silt were studied according to 59 fiber length and fiber content. The research findings are expected to serve as a reference for applying 60 fiber-stabilized carbide-slag-solidified soil in road engineering. It can protect the environment, 61 improve the resource utilization efficiency of carbide slag, reduce the project cost, achieve 62 sustainable development, and achieve remarkable economic and social benefits.  The soil samples used for the tests were collected from the Airport, New Area of Langfang, 67 Hebei Province, China. The study did not involve private or protected land. According to Test 68 Methods of soils for Highway Engineering (JTG 3430-2020) [20], the physical indexes and particle 69 gradation of test soil are determined. The physical indexes are listed in Table 1, and the particle size 70 distribution is shown in Fig 1,

76
Carbide slag was purchased from Dezhou Shihua Chemical Co., Ltd. The raw calcium 77 carbide slag had a high moisture content and a peculiar smell. After drying and dehydration, it was 78 crushed by soil milling and passed through a 0.075 mm square hole sieve. The carbide slag under 79 the sieve was used for the tests (Fig 2). The main components of the calcium carbide slag are listed 80 in Table 2. The carbide slag contained a large amount of calcium oxide similar to that of quick lime 81 and exhibited significant activity (  The fibers were purchased from Changsha Ninxiang Building Materials Co., Ltd. 85 Polypropylene fibers with lengths of 6, 12, and 19 mm were used for testing (Fig 3).  Table 3. 89

92
This study was divided into three steps to investigate the effects of the fiber length and 93 content on the performance of fiber-stabilized carbide-slag-solidified silt, optimal fiber length and 94 content, and effect of fiber distribution density on the mechanical properties of the solidified soil. proportionally added to the mixture and thoroughly stirred manually to fully disperse the fiber into 111 the mixture. An appropriate amount of water was added to the mixture blended with the fiber, stirred 112 evenly, and placed in a plastic bag. The bag was sealed and placed in the dark for 24 h. A 50 mm × 113 50 mm test mold was adopted, and the calculated weight of the soil sample was determined based 114 on the degree of compaction (96%). Twelve samples were prepared for each group. After the 115 specimens were produced using the static pressure method, they were placed in a curing box 116 (temperature = 20±2 °C; relative humidity ≥ 95%). The specimens are shown in Fig 4. The 117 proportions of the specimens of fiber-stabilized carbide-slag-solidified soil are listed in Table 4. According to the experiment, the optimum moisture content of 2:8 carbide-slag-solidified 132 soil was 13%-14%. First, five portions of 2:8 carbide-slag-solidified soil were prepared, and the 133 target moisture contents were 10%, 12%, 14%, 16%, and 18%. Compaction tests were performed 134 according to the compaction test method for inorganic binder stabilization materials (T0804-1994). 135 The compaction curve of the soil samples is shown in (Fig 5). The optimum moisture content of 2:8 136 carbide-slag-solidified soil was 13.62 %, and the maximum dry density was 1.68g/cm 3  The failure pattern of the specimens subjected to unconfined compression is shown in Fig  144   6.

181
Because fibers are stable materials, they do not react with silt or calcium carbide slag, and 182 their effect on soil is similar to the mechanism by which plant roots solidify in soil. According to 183 the bending mechanism, the bending fibers between soil particles limit the change in the relative 184 position of the soil particles through the pressure and frictional force on the concave side of the soil 185 particles, stabilizing the soil. According to the intercrossing mechanism, the intercrossing fibers in 186 soil form a spatial network structure, and the action of external forces in one direction is 187 counteracted by the entire network structure. In addition, the incorporation of fibers effectively 188 reduces the internal pores of solidified soil, densifies the soil, and improves its mechanical 189 properties; this can be called the "filling mechanism." Based on the bending, intercrossing, and 190 filling mechanisms, the preconditions for fiber stability in the soil are that the fiber must have a 191 sufficient length and minimum distribution density in the soil. cementation reaction between the carbide slag and soil particles occurred, and the newly generated 214 reaction products adhered to the spatial network structure formed by the fibers, limiting soil 215 deformation and increasing the unconfined compressive strength of the soil. For the same carbide 216 slag content and soil quality, the intercrossing mechanism effect became more significant with 217 increasing fiber content and length. Moreover, the spatial network structure stabilized further, and 218 the increase in the unconfined compressive strength of the solidified soil became significant. 219 However, when the fiber content exceeded a specific value, the fibers were less uniformly 220 distributed in the soil, and the spatial network structure became less stable.  Under the same conditions, the unconfined compressive strength of the 7 d solidified soil 263 slightly increased with the fiber content (Fig 10). The unconfined compressive strength of the 264 solidified soil reinforced with 19 mm fibers was the highest but only 7.14% higher than that without 265 fibers. The main reason for this was that for the short curing period, the fibers did not form an 266 effective bond with the solidified soil, and the bending and intercrossing actions were somewhat 267 challenging to achieve. The increased unconfined compressive strength was mainly attributed to the 268 effective reduction of the pores in the solidified soil by the incorporated fibers and the resulting 269 filling effect, densifying the solidified soil. The solidified soil reinforced with 19 mm fibers using 270  Commented [zh31]: All "interleaving effects" had been changed to "intercrossing actions".

Commented [zh32]
: " suggests that" change to " revealed that" (1) The fibers did not significantly increase the unconfined compressive and indirect 295 tensile strengths of the carbide-slag-solidified soil at 7 d but effectively increased those at 28 d. 296 When 19 mm long fibers were incorporated into the carbide-slag-solidified soil, the effect became 297 most significant for the fiber content of 0.3%. 298 (2) The increase in the indirect tensile strength of the carbide-slag-solidified soil by the 299 fibers was significantly higher than that in the unconfined compressive strength. 300 (3) For short-period (7 d) curing, the stabilization of the carbide-slag-solidified soil by the 301 fibers mainly depended on the filling action. For long-age (28 d) curing, the stabilization by the 302 fibers was caused by the combined actions of filling, intercrossing and bending. 303 304